電磁超音波計測による金属疲労の研究 : き裂発生と転位組織変化の非接触モニタリング

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This study is devoted to contactless monitoring of attenuation and nonlinearity of the surface-shear wave during rotating bending fatigue of carbon steels for predicting the remaining life. The attenuation showed a sharp peak at 85pct of the total life. The nonlinearity showed two peaks at 60 and 85pct of the total life. These peaks appeared at fixed fractions of lifetime, being independent of the bending stress (0.49-1.20 of the yield stresses) and the carbon content (0.22-0.45mass%). We attribute the earlier nonlinearity peak to the crack nucleation and growth; and the attenuation peak and the later nonlinearity peak to an increase of free dislocations associated with crack extension in the final stage. Microstructure observations with TEM, surface crack study with replicas, and the acoustic measurements show that a large-scale change occurs in the dislocation structure (persistent slip bands to cells) at the attenuation peak and that it is triggered by the inward growth of cracks. This change completes in a short time, a few percent of the total lifetime. The acoustic-resonance technique can be an important means for the exact prediction of the remaining life of fatigued steels.